In the processing of semiconductor substrates, plasma processing is often employed. Plasma processing may involve different plasma-generating technologies, for example, inductively-coupled plasma processing systems, capacitively-coupled plasma processing systems, microwave-generated plasma processing systems, and the like. Manufacturers often employ capacitively-coupled plasma processing systems in processes that involve the etching of materials using a photo resist mask.
Important consideration for plasma processing of substrates involves a high etch rate and a high photo resist selectivity. A high etch rate refers to the rate at which the target material is etched during plasma processing. Generally speaking, the faster the underlying layer can be etched, a greater number of wafers can be processed per unit of time. All things being equal, manufacturers desire to process more wafers per unit of time to increase wafer processing efficiency. Photo resist selectivity refers to the discrimination between the photo resist mask and the underlying target layer during etching.
As circuit density increases, manufacturers are required to etch or to form a greater number of devices per unit area on the wafer. The higher device density requires a thinner photo resist layer. The thinner photo resist layer, in turn, tends to be more susceptible to being inadvertently etched away. As a result, manufacturers constantly strive to create processing recipes that can etch the underlying layer at a high etch rate while avoiding damage to the photo resist mask.
One way to increase the etch rate is to increase the plasma density during plasma processing. In a capacitively-coupled plasma processing system, plasma density may be increased by increasing the power of the higher frequency RF signals. To facilitate discussion, FIG. 1 shows a prior art multi-frequency capacitively-coupled plasma processing system 100, representing the plasma processing system typically employed to process substrates. As seen in FIG. 1, multi-frequency capacitively-coupled plasma processing system 100 includes a chamber 102 which is disposed in between an upper electrode 104 and a lower electrode 106.
In the implementation of FIG. 1, lower electrode 106 is provided with multiple RF frequencies, such as 2 Megahertz, 27 Megahertz, and 60 Megahertz. Upper electrode 104 is grounded in the implementation of FIG. 1. Multi-frequency capacitively-coupled plasma processing system 100 also includes a plurality of confinement rings 108A, 108B, 108C, and 108D. The confinement rings 108A-108D function to confine the plasma within chamber 102 during plasma processing.
There is also shown in FIG. 1 a peripheral RF grounded ring 10, representing the RF ground for the plasma generated within chamber 102. To isolate peripheral RF ground 110 from upper electrode 104, an insulating ring 112 is typically provided. A similar insulating ring 114 is also provided to insulate lower electrode 106 from an RF ground 116. During plasma processing, the RF power provided to lower electrode 106 excites etching gas provided into chamber 102, thereby generating a plasma within chamber 102 to etch a substrate that is typically disposed on lower electrode 106 (substrate is not shown to simplify FIG. 1).
As discussed earlier, it is highly desirable to etch the target layer on the substrate while the substrate is disposed in chamber 102 without unduly damaging the overlying photo resist mask. In the prior art, increasing the etch rate of the target layer may be achieved by increasing the plasma density within chamber 102. Generally speaking, the plasma density may be increased by increasing the power level of the higher frequency RF signals that are provided to lower electrode 106. In the context of the present invention, a high frequency RF signal is defined as signals having a frequency higher than about 10 Megahertz. Conversely, RF signals with frequencies below 10 Megahertz are referred to herein as Low Frequency Signals.
However, by increasing the power level of the higher frequency RF signals (e.g., the 27 Megahertz RF signal or the 60 Megahertz RF signal of FIG. 1), it may be challenging to confine the generated plasma within chamber 102. Even if the plasma can be satisfactorily confined, electron loss to the upper electrode during plasma processing places an upper limit on the plasma density within chamber 102. It has been found that as the plasma density increases; electrons are lost to the grounded upper electrodes or other grounded surfaces of the multi-frequency capacitively-coupled plasma processing system 100, thereby causing the plasma density within chamber 102 to reach a saturation point. Beyond this saturation point, increasing the RF power of the higher frequency RF signal does not increase the plasma density since the electron loss outpaces the generation of ions.
Furthermore, increasing the RF power to the higher frequency RF signals has been found to adversely affect the photo resist selectivity. At a high RF power level, the photo resist mask is damaged to a greater extent due to increased bombardment, which causes the photo resist mask to erode away at a faster rate, thereby negatively impacting the etching process.
FIG. 2 shows a prior art implementation whereby one or more high frequency RF signals 202 (e.g., the 60 Megahertz RF signal of FIG. 2) are provided to upper electrode 104 in order to provide additional control over the generation of ions within chamber 102. However, the implementation of FIG. 2 still does not solve the aforementioned problem of plasma density saturation point effect. When the RF power level of the higher frequency signal provided to upper electrode 104 is increased, the aforementioned saturation point effect is also observed, limiting the plasma density and consequently, the etch rate through the target layer irrespective of the increase in the RF power level to the higher frequency RF signals.
Additionally, other prior art implementation has tried to control the photo resist selectivity by controlling the temperature of the electrodes. It has been found that the approach of controlling the temperature of the electrodes is minimally effective in controlling the photo resist selectivity. Furthermore, the approach of controlling the temperature of the electrodes does not address the aforementioned problem of plasma density saturation point effect.
Therefore, various aforementioned prior art implementations have proven ineffective in increasing etch rate without adversely affecting or maintaining high photo resist selectivity in capacitively-coupled plasma processing system in processes that involve the etching of materials using a photo resist mask. In the prior art implementation of FIG. 1, the increase in RF power to the lower electrode may lead to unconfinement of plasma, saturation point of plasma density, and adversely affect the photo resist selectivity. Whereas in the prior art implementation of FIG. 2, the increase in RF power to the upper electrode may lead to saturation point of plasma density. Furthermore, the prior art implementation of controlling temperature of the electrodes is minimally effective in controlling the photo resist selectivity while providing no solution for the plasma density saturation effect.